Bray Hill is an unremarkable residential street with little more than a crossroads and a newsagent. But tonight, an expectant crowd has gathered. Many arrived two hours ago to secure a position by the side of the road; two young boys have climbed a telephone box for a better view.

It's Friday-night practice at the Tourist Trophy - a week-long series of motorbike races known as the TT, the most dangerous road race in the world. Bray Hill, in Douglas, the Isle of Man's capital, is a couple of miles from the start of the course and offers one of its most dramatic vantage points. When the first of the 1,000cc Superbikes pass, mere yards away, the speed takes you by surprise. There is a metallic crunch when the bike hits the bottom of the hill; five seconds later, it has disappeared out of sight.

After half an hour, there is an announcement: three riders have crashed at the section of the course known as Black Dub and a fuel tank has set fire to a hedge. The riders' injuries are relatively light: a broken leg, a broken ankle, a damaged shoulder. The fire is put out and practice restarts.

For the 50,000 fans who travel each June to this island in the Irish Sea, danger is part of the appeal. The time trial takes in 37 miles of the island's roads and some of its most verdant scenery. By the start of this, its centenary year, 223 riders had died on the TT course; it would be 224 before the end of the week. Marc Ramsbotham, a 34-year-old newcomer, collided with spectators on 8 June, killing two onlookers as well as himself. For most of the week, it had seemed that the tournament's celebratory year would be free from fatalities; Ramsbotham crashed in the very last stages of the final race.

The TT inspires more passion and loyalty than any other motorcycling event. Compared to the short, sanitised circuits of MotoGP, this is tough racing on real roads, with dozens of sequences to remember. The blind corners and the steep mountain climbs of Snaefell require years of experience to ride well; handling a 200kg bike around them at average speeds close to 130mph is gruelling, not to mention nerve-shredding. Should your concentration slip, there are no gravel traps to break your fall. There are only trees, telegraph poles and jagged stone walls.

On Saturday afternoon - with the blue-riband Superbike race postponed because of mist on the mountain - John McGuinness sits in his Honda trailer and explains how the TT captivated him as a boy. 'It's like everything you wanted to do on a road bike but you can't do or you'd go to jail,' he says, smiling. Now, at 34, he dominates the event. He has won 11 TT races and this year the talk is of whether he can record the first ever 130mph lap. 'Pretty good for a fat brickie from Morecambe,' he says.

He has competed in the event for a decade, yet says: 'I still have fear. I've seen some nasty things around here and I do say my prayers before I go out. But I'm more scared of not doing any good, that's what worries me most.'

McGuinness is one of a handful of professionals in the race. Most of the 70 starters are, at best, semi-pro; some pitch up with just a tent and a bike, having saved money all year to compete. McGuinness's closest rival, Guy Martin, is a 24-year-old engine-fitter from Lincolnshire. 'I like getting up early and going to work,' he says. 'You're covered in muck and you're knackered and it makes you hungrier to go and win.

'Everybody does the TT for different reasons. The big thing for me is the buzz. I've tried all sorts of stuff - snowboarding, skiing, downhill mountain biking - but for getting the same buzz there's nothing. You're completely in control and if anything goes wrong, nine times out of 10 it's your fault. OK, it's bloody dangerous, but we all know the risk. More people get killed every year at Everest, and they never make anything of that.'

'Mad Sunday' is when fans ride the course themselves, often in fancy dress. This year, heavy rain has put many off; but on the roadsides, people still turn out to cheer bedraggled riders in superhero outfits. Down by Douglas's seafront, a stunt competition is ruined because of the wet tarmac; instead, people drift down the promenade to hundreds of parked bikes, gathering around the snazziest models.

By the morning of the rescheduled Superbike race, the rain has passed. The pub at the Creg ny Baa corner has erected two grandstands, but most spectators choose to sit along the grass banks, closer to the action. Even here, where you can see for half a mile in either direction, the TT is not a good spectator sport. It's not just the personal risk and the long delays: with the racers starting at 10-second intervals, the simplest overtaking manoeuvre is a rare sight.

What it is, instead, is a social event. John McGuinness says that even travelling at 200mph he can smell the barbecues by the side of the course. 'You think, "Bastards, they're having sausage and burgers and I'm flying down here at 200mph, stressed to death."'

Fans from as far away as New Zealand return each year for the craic. John, a 54-year-old Canadian, has been 10 times. 'Sure, you don't see much of the racing,' he says, 'but there's nothing quite like this atmosphere. Everybody talks the same language. If you're a biker, you know exactly what they're doing out there. You've got total respect for the sheer guts of the riders.'

Former winners are treated like returning war heroes. Richard Quayle - nicknamed 'Milky' because of his endearing resemblance to the Milkybar Kid - is one of only three Manx men to win a TT title and, as such, is the best-loved taxi driver on the island. Quayle drives me past the scene of his 2003 accident, one of the most famous crashes in TT history. A video exists of the moment that he caught his shoulder on a wall at 160mph and was thrown down the road, bouncing from one side of it to the other. You can see it on YouTube: search for milky + crash. Quayle punctured both lungs and had to have his spleen removed. His wife was pregnant at the time: the TT career had to end.

In the paddock, another former winner, Tony Jefferies, is trying to get from the grandstand to the beer tent. It's not his wheelchair that makes it hard; it's that every time he gains a few yards, someone stops him for a chat. The Jefferies family are well known here; Tony and his father were top riders and Tony's son, Dave, became a TT icon when he won an unprecedented nine titles in three years. In 2003, at his peak, Dave lost control on an oil patch during practice and was killed.

McGuinness, his best friend, arrived at the scene seconds later. He said the sight was reminiscent of a plane crash. 'To be honest I just wanted to go home,' he says now. 'The strongest person was Pauline, David's mum. She said you must carry on riding, be strong for all the riders. So I went out at 5am the next morning and went through the same corner David crashed in and said to myself, "Look after me, David." I still say that every time I go through that corner.'

Not for the first time, the TT's future looked in doubt; attendances dropped and manufacturers were wary of being involved. But McGuinness's flair has reinvigorated the event; this year, as expected, he wins the Superbike race and the Senior TT. Tony and Pauline still come every year. 'We only ever stay for a couple of days,' says Tony. 'Unfortunately, in racing you get used to death. If you're thinking about the dangers when you go and race you shouldn't be riding. Go and play golf.'

As he talks, a stranger from Ireland approaches. He has never met Tony; never even met Dave; but he wants Tony to know that Dave is never forgotten by 'the boys back home'. When he leaves, there is silence. Tony wipes his eyes; Pauline, too, is tearful. 'But if I could do it all again I'm afraid I would do,' she says. 'Because that boy was happy.'